Abstract: This proposal will examine the role of cellular positioning in the establishment of synaptic connectivity. Toward this end, experiments will be carried out in a mouse model of a human neurodevelopmental disorder, lissencephaly. Lissencephaly affects approximately 1 in 100,000 live births and mainly results from mutations in genes essential to cell proliferation and migration; the most common of which and first identified is Lis1. In lissencephaly patients, like other microcephalies, cells fail to properly migrate during embryonic development, resulting in heterotopic cell clusters. In this mouse model, heterotopic cell bands form instead of the single principal cell layer found in the normal hippocampus. Experiments in Aim 1 will determine if heterotopic bands share common features, which might suggest the existence of discrete excitatory cell types in the non-mutant hippocampus, or alternatively may indicate that heterotopic band segregation appears to be random. Several methods will be employed in this first aim, including retrograde bead injections, immunohistochemistry, and in vitro whole-cell recordings with cellular morphological reconstructions. In either outcome, experiments in the Aim 2 will use this model as a tool to investigate the effect of cellular positioning in the determination of synaptic partners by assaying interneuron connectivity within and between heterotopic bands. Recordings will be made from identified interneurons while glutamate is uncaged on cells occupying various heterotopic bands to examine the spatial distribution of synaptic innervation. These experiments will yield detailed information about synaptic topology that can then be post-hoc aligned with heterotopic band locations and excitatory cell identity. These findings will add to our understanding of the human population carrying the Lis1 mutation, and also more broadly to cellular heterotopias in other neurodevelopmental disorders. Recent high-profile journal publications indicate that excitatory cell sub-types might exist in the radial axis of the hippocampal principal cell layer, a possibility that was previously impractical to study for technical purposes. A greater understanding of canonical forms of connectivity between specific subsets of excitatory and inhibitory cell populations seems poised to usher in a new wave of scientific progress in the field of neuroscience. This may reveal the existence of dedicated circuitry for specific brain functions, offering new therapeutic targets and simultaneously reducing off-target effects. Microcephalies have garnered renewed attention from the public owing to the spread of the Zika virus, which can cause microcephaly in babies exposed to the virus embryonically. In order to improve the quality of life and long-term prognosis for these patients the scientific community needs a deeper understanding of how disorders of development and cell migration influence synaptic connectivity.